In the past, rock "slicers" and machines for "cutting" a rock with pressure tools, with the ability to transduce and a monitor to display screen the performance and position of the cutting blades were known in the art. The concept of a traveling rotary tool is disclosed in my U.S. Pat. No. 3,614,161 in which the tool discharges reactions caused by the action of excavation almost exclusively in the transverse direction to the main axis through a guide and against one or other lateral walls of the excavation. In Sherard, U.S. Pat. No. 3,645,301, the various forms of downwardly operating rock excavating tools are disclosed in conjunction with a movable rigid barrier for separating the initial slurry-filled starter hole or trench so that fluid columns of the slurry are formed on the front and rear sides of the barrier. In each of these techniques, the trench is maintained full of an excavating slurry, typically a mixture of bentonite, clay or mud. In each case, the excavation is from an upward direction downwardly upon the soil or earth or rock to be excavated. These prior art excavation systems use reverse circulations as well as air lifts for removing the excavated material from the trench.
According to the present invention, a rock mill is provided for removing rock by vertical increments from the bottom of the trench upwardly while operating from a full depth "starter" hole and can be withdrawn from the excavation quite easily. This allows the excavated material to fall away from the cutting edge thereby improving the production rate and disassociating the excavation rate from the removal of the spoils. It also utilizes previously poured panel or, in the preferred embodiment, a precast panel which has been lowered into the excavation to discharge part or all of the reaction it needs to apply pressure to the cutting tool so that the resulting equipment can be made lighter and simpler and more maneuverable. Moreover, by using as a starter hole the joint of a previously poured or installed concrete panel, the invention can guarantee full section, continuity of the wall joint and not just the minimum contact (typically about 18").
Moreover, the invention avoids a critical draw back of all tools which remove rock in a downward mode namely, the removal of spoils. For a cutting tool to be efficient all chips of rock have to be immediately removed from the point of advancement of the cutting teeth of the tool otherwise they will be reground by the cutting teeth while imposing a continuous risk of wedging the cutting wheel. This is generally accomplished by direct or reverse air fluid circulation. For the circulation to be efficient the quantity and pressure of air or fluid has to be substantial which results in high cost and potential stability dangers. In a preferred embodiment of this invention, a conventional air lift at the bottom of the starter hole is utilized.
Furthermore, rock penetration is a function of the weight on the cutting teeth and of the stability of the machine. The preferred embodiment of the invention utilizes in part the horizontal reaction of a previously poured concrete section or the precast section which has been lowered into the trench to apply pressure on the tool and remains firmly positioned against the cutting surface being guided by the joint.
Thus, a significant feature of the method aspects of the invention is in the idea and concept of excavating a trench slot in rock from the bottom up rather than from the top down. It is easier and less expensive to drill a large shaft from the bottom up rather than in a downward mode.
Another advantage of a preferred embodiment of the invention is that a hydraulically operated expandable or extendable milling wheel permits an excavation by undercutting of existing structures which may be a very significant advantage in the work on a main dam under an outlet structure, for example. Moreover, this method allows for easy contact between the slurry wall and any existing concrete surface even in inclined ones since it can easily regulate the amount of penetration of the cutting tool into the concrete.
Where the overburden is conventional soils, excavation will proceed through the embankment and core materials with traditional slurry wall clam shells, backhoe and excavation in rock will start where the excavation by clam shells end.
In the initial excavation, which can as noted above, be by means of conventional clam shells or by backhoe, the excavated trench is filled with a lean bentonite-slurry mix thus eliminating the need of excavating with a rock milling machine at or near the surface and giving the possibility of a better guide to the tools on the surface. Starter panels are utilized which allow the start of the chain of excavation in both directions. Once the excavation of the starting slot is completed, the bottom is cleaned and the bentonite slurry is desanded and an amount of cement-bentonite is placed at the bottom of the trench. When the trench is ready, a series of quadra-concave starter precast concrete elements, which may be steel reinforced, stacked one on top of the other, are lowered into the trench in such a way as to leave a width of unobstructed, grout-filled trench at each side of the stacked precast elements. The stacked precast elements are automatically aligned with respect to each other by a tongue-and-groove system at each horizontal interface. The horizontal seal between the individual precast elements is guaranteed by a sealant such as a bentonite sausage sandwiched between the upper and lower elements spanning from one vertical groove to the other. This bentonite sausage is manually placed in the groove of the panels while installing them.
These precast elements are placed for the entire depth of the excavation. Both the upstream and downstream concave bases are simultaneously filled with plastic tremie concrete which will displace the grout and permanently stabilize and integrate the precast elements with the earth fill embankment and the rock. The unique shape and construction of the precast elements is such that they will create a guide from which to continue the trenching operations while minimizing the possibility of concrete flowing into the adjacent chambers or trench sections.
When the starter panel is in place, a single axle rock or swaying mill cleans the grout and the guide trench until the bottom of the excavation is reached. Cuttings are excavated by slurry circulation. After the cleaning operation is completed, the single axis rock mill continues the excavation exploiting as many capabilities in upward, horizontal or downward direction. This operation is effectively guided by the sides of the previously cut trench as well as by the precast element already in place thus insuring continuity and consistency with the vertical alignment of the precast concrete elements already in place. The excavation of the cuttings will depend on the direction of the excavation of the cutting mill and will either be continuously excavated by a slurry circulation, air left at the bottom or will be taken out once the trenching is completed by a mechanical clam shell. When the trenching and the cleaning of the bottom is completed, the guiding opening of the previously cast concrete elements is cleared of grout by a correspondingly shaped tool and the bentonite in the trench is now lifted from the bottom by an air lift and is circulated through the desander until the qunatity of sand in suspension is brought within a predetermined percentage (2%, for example). This operation always sweeps the bottom of the excavation clean and is routinely performed unless the system used to excavate the cuttings was such as to warrant a clean bottom of the excavation and a maximum percentage of sand content within the limits previously set.
When the excavation is ready a predetermined volume of cement bentonite grout is pumped to the bottom of the excavation and as soon as the grout is placed, a series of precast concrete panels stacked one on top of the other is lowered into the excavation. This column of stack of precast elements have convex ends which interengage and fit within the concave ends and have opposite concave ends similar to those in the starter precast concrete elements. The concave sides of those elements make them light-weight and their shape maks them easy to use. This vertical chain of precast panels is horizontally aligned by a tongue-and-groove system in the precast panels while its vertical alignment is insured by a temporary continuous guide placed into the opening existing in the previously cast element. This temporary continuous guide is lowered at the same rate of the precast panel to the bottom of the excavation thus interlocking the previously placed precast panels to the elements being placed. As noted above, squeezed into the tongue-and-groove recesses is a bentonite sausage which insures the water tightness of this connection. As the stack of precast elements reaches the bottom, the suspending rods and the temporary continuous guides are withdrawn and a plastic tremie concrete is then preferably simultaneously placed into the upstream and downstream concave spaces thus stabilizing and integrating the precast elements with the earth fill embankment in the rock. The process is sequentially repeated to achieve a chain of stacked precast elements to form the wall and panel sections.
This new technology automatically insures:
1) Full continuity of the trenching since each excavation is the guided extension of the previous one;
2) It allows direct visual inspection of the quality, soundness and dimensional fitness of each precast panel thus guaranteeing the same quality throughout the wall;
3) The plastic concrete placed upstream and downstream of the precast elements although of high quality, does not determine the water tightness of the wall which is entrusted to the bentonite sausage for the horizontal joints, the cement grout for the vertical joints and the precast panels themselves for the majority of the surfaces and the concrete in the concave spaces;
4) Virtually all of the elements determining the water tightness of the wall are visually inspected while being installed and the vertical joints are individually checked hydrostatically before being sealed with grout;
5) Unique working features of the single axle rock or swaying mill that allows to excavate in a upward, and horizontal direction and downward as well, and enables it to be easily withdrawn from the excavation. Moreover, the rockmill with proper teeth orientation can be rotated in either direction.